The present invention relates to a sliding type tripod constant velocity universal joint applicable to a power transmission unit in vehicles, aircrafts, vessels, and various industrial equipment. Generally, a constant velocity universal joint is a type of universal joints for connecting two shafts, a driving shaft and a driven shaft, and capable of transmitting rotational force at a constant velocity even if there is an angle between the two shafts. A sliding joint is a joint which allows relative displacement of the two shafts in the axial direction by the plunging of the joint. A tripod type joint is designed to transmit torque between two shafts by coupling a tripod member having three trunnions protruding in the radial direction to one of the shafts, while coupling an outer joint member with a hollow cylindrical shape provided with three track grooves extending in the axial direction to the other shaft, such that the trunnions of the tripod member are received in the track grooves of the outer joint member.
A tripod type constant velocity universal joint is an example of constant velocity universal joints used as means for transmitting rotational force from a vehicle engine to wheels at a constant velocity. The tripod type constant velocity universal joint is constructed to couple two shafts, a driving shaft and a driven shaft, such that rotational torque can be transmitted at a constant speed even when a working angle is formed by the two shafts, and their relative displacement in the axial direction is allowed.
In general, a tripod type constant velocity universal joint is principally constituted by an outer joint member having three track grooves formed in the inner circumference to extend in the axial direction, and roller guide surfaces extending in the axial direction on the opposite sides of each of the track grooves; a tripod member having three trunnions protruding in the radial direction; and a roller accommodated rotatably between each of the trunnions of the tripod member and the corresponding roller guide surfaces of the outer joint member. One of the two shafts is coupled to the outer joint member and the other is coupled to the tripod member.
The trunnions of the tripod member and the roller guide surfaces of the outer joint member are thus engaged with each other via the rollers in the rotational direction of the two shafts, whereby the rotational torque is transmitted from the driving shaft to the driven shaft at a constant speed. In addition, each of the rollers rolls on the roller guide surfaces while rotating with respect to the trunnion, whereby relative displacement in the axial direction and relative angular displacement between the outer joint member and the tripod member are absorbed.
One type of such a tripod type constant velocity universal joint is configured such that a roller is mounted on the outer peripheral surface of a trunnion with a plurality of needle rollers interposed therebetween. Since the roller and the roller guide surface are positioned obliquely to each other along with the inclination of the trunnion when rotational torque is transmitted with a working angle being formed between the outer joint member and the tripod member, the roller will slip with respect to the roller guide surface to inhibit the smooth rolling of the roller, posing a problem of increasing the induced thrust. Also, the frictional force between the roller and the roller guide surface will increase the sliding resistance between the outer joint member and the tripod member during their relative displacement in the axial direction.
It should be noted that the “induced thrust” refers to the thrust force that is generated by friction inside a constant velocity universal joint when torque is applied to the joint at a certain angle during rotation thereof. Typically, this rotational force occurs strongly as a tertiary component in a tripod type joint. Also, the “sliding resistance” refers to a magnitude of the axial frictional force generated when an outer joint member and a tripod member slide with respect to each other in a sliding joint such as a tripod type constant velocity universal joint.
For the purpose of solving the problem that a roller is positioned obliquely to a roller guide surface, and reducing the induced thrust and the sliding resistance, there have been proposed various types of tripod type constant velocity universal joints having a roller mechanism that allows a roller to tilt or be axially displaced freely with respect to a trunnion. One of such known tripod type constant velocity universal joints is configured such that a roller is mounted rotatably to a ring with a plurality of needle rollers interposed therebetween to constitute a roller mechanism (roller assembly) and the inner peripheral surface of the ring is formed such that the cross section is of a circular convex shape and fitted externally on the outer peripheral surface of the trunnion (see FIG. 11(A) of Japanese Patent Laid-Open Publication No. 2000-320563, for example). According to such a configuration, the slip between the cylindrical inner peripheral surface of the ring and the outer peripheral surface with a convex spherical shape of the trunnion allows the roller mechanism to tilt and be axially displaced with respect to the trunnion, and therefore the roller can be prevented from being positioned obliquely to the roller guide surface.
Additionally, the cross section of the trunnion is formed into such a shape, elliptic shape for example, that allows the trunnion to contact the inner peripheral surface of the ring in the direction perpendicular to the axis of the joint and forms a gap between the trunnion and the inner peripheral surface of the ring in the axial direction of the joint (see FIG. 1(B) of Japanese Patent Laid-Open Publication No. 2000-320563, for example). In this manner, it is possible to allow the trunnion to tilt with respect to the outer joint member without changing the attitude of the roller assembly when the joint is set with a working angle. Furthermore, since the contact ellipse between the ring and the outer peripheral surface of the trunnion transforms from an ellipse that is long sideways towards a point, the frictional moment acting to tilt the roller assembly is reduced. As a result, the attitude of the roller assembly is allowed to be always stable, and the roller is held parallel to the roller guide surface and hence is allowed to roll smoothly.
In the meantime, the induced thrust and the sliding resistance as mentioned above are responsible for vibration or noise of a vehicle body, affecting the NVH characteristics of the vehicle, and reduce the design freedom of the underbody of a vehicle. Therefore, it is desirable to minimize the induced thrust and sliding resistance as much as possible.
Specifically, in this type of tripod type constant velocity universal joint, when torque is transmitted with an angle, the mutual friction between internal components of the joint causes the induced thrust when the joint is rotating, and when not rotating, the slide resistance if the joint is forcibly extended or contracted in the axial direction. Typical the Noise Vibration Harshness (hereinafter referred to as “NVH”) phenomena of a vehicle involving the induced thrust and the sliding resistance include rolling of a vehicle body during traveling, which is caused by the former, and idling vibration at the drive or D range during halting in an automatic transmission vehicle, which is caused by the latter.
For solving the NVH problems in vehicles, it is important to minimize the magnitude of induced thrust or sliding resistance of the joint. In general, the induced thrust or the sliding resistance tends to depend on the magnitude of the working angle. Therefore, when the joint is employed for a drive shaft of a vehicle, restriction is imposed on the design such that the working angle cannot be made large. In order to increase the degree of design freedom of the underbody of a vehicle, there has been problem to stabilize the induced thrust or the sliding resistance at a low level.
On the other hand, in this type of constant velocity universal joint, some wear occurs in the roller guide surface of the outer joint member after its endurance life has expired. In a constant velocity universal joint exhibiting low vibration performance, particularly, there has been a problem that, if wear occurs on the roller guide surface of the outer joint member after expiration of the endurance life, it becomes difficult to reduce the induced thrust due to such wear, and thus the NVH characteristics are deteriorated remarkably.